Feature request: UI-based CSV/JSON bulk import for workspace admins

### Description of the issue

Feature request: add a UI-based bulk import flow (CSV/JSON) for workspace admins in Huly.

Today, import is available via CLI/Docker tooling (`dev/import-tool`) and requires technical setup + admin credentials. There is no equivalent end-user import UI in the app (while export has UI support via `export-resources`).

This makes onboarding harder for non-technical teams, especially on Huly Cloud.

### Your environment

* Version of Huly: Cloud (`huly.app`)
* Browser (and version): Chrome (latest)
* Your operating system (and version): macOS (latest)

### Steps to reproduce

1. Open Huly Cloud and try to import workspace data from CSV/JSON in Settings or project/workspace UI.
2. Observe that export actions are available in UI, but import actions for workspace data are not.
3. Current documented path requires CLI/Docker import tooling.

### Expected behaviour

A built-in import UI for authorized users (workspace admins / users with import permission), including:

- Select source type (CSV/JSON, and optionally predefined mappers)
- Upload file(s)
- Validate + preview mapped fields before applying
- Run import asynchronously with progress and result summary
- Downloadable error report for failed rows

### Actual behaviour

No UI-based bulk import flow is available for general workspace data.
Imports are effectively one-time CLI-driven operations.

### Possible solutions

Suggested incremental approach:

1. MVP: UI upload + validation/dry-run preview (no writes)
2. Async import job endpoint + progress status + notifications
3. Final apply with permission checks, row-level error reporting, and audit logs

Security/operational considerations:

- Permission-gated (import permission)
- File size/type limits
- Rate limiting + quota controls
- Idempotency / duplicate handling options
- PII-safe error reporting